Sept. 16, 2004 -- Researchers say that increasing numbers of girls experiencing puberty earlier may not be the result of the rise in childhood obesity.
It has been suggested that the trends seen in girls beginning puberty early is driven by the increase in childhood obesity.
But researchers from the Lifespan Health Research Center at Wright State University School of Medicine in Dayton, Ohio, say that an increase in weight seen during this time is a consequence of puberty rather than a driving force.
Several studies have noted a drop in the age at which a girl first gets her period. Recent studies have shown that girls have started menstruating on average two to three months earlier. Significant declines are especially seen in some ethnic groups such as blacks, this finding was concurrent with the increases in BMI seen in children. This has prompted speculation that the rising epidemic of obesity might be to blame for the increasing trends in early puberty.
The increase in body fat might accelerate the age of puberty because fat can get converted into the sex hormone need for sexual development.
Currently, about 15% of all American children aged 6-19 and more than 27% of black girls aged 12-19 are overweight, according to the researchers.
A Half Century of Information
To investigate the possible link, associate professor Ellen Demerath, PhD, MA, and colleagues reviewed 50 years of data.
They focused on the age at which these girls first had a period and BMI (an indirect measure of body fat). The study included 211 white American girls born between 1929 and 1983.
The girls followed by Demerath's team were part of the Fels Longitudinal Study. From age 2 to 18 years, the girls made semiannual visits to Fels researchers.
Starting at age 9, the girls were asked if they had started having periods.
Early maturers was defined as a girl who had her first period no later than age 11.9, average maturers started menstruating between 12 and 13.1 years, and late maturers had periods that occurred after age 13.2
The girls who started menstruating early had a greater BMI increase during adolescence than those who started menstruating at average or late ages.
However, the BMI increase didn't happen until after a girl's first period.
"Although girls with early menarche eventually became heavier than other girls, they were not heavier than average-maturing and late-maturing girls during their prepubescent period," write the researchers.
Cause and Effect
The findings show that early menstruation might have led to increased BMI -- not vice versa, as others have suggested.
"The causal arrow seems to lead from earlier menarche toward increased BMI," write the researchers.
The average age of first period was "highly stable" throughout the 50-year time frame, say the researchers.
It's too soon to tell what, if any, differences will be seen in girls born after 1983.
Changes in menarche age aren't unique to modern America. Western Europe had a four-month drop in average menarche age between 1830 and 1950, say Demerath and colleagues.
The study appears in the Aug. 1 issue of The American Journal of Clinical Nutrition.